From US2006/0196450 A1 (Rocco Giannoni) a heat exchanger is known which comprises a coil shaped heat exchanger pipe assembly which is received in a housing. The objective of that publication was to be able to provide a heat exchanger whose housing is made from plastic instead of from metal, so that the heat exchanger as a whole can be made of lighter weight. As described in the publication in question, the coil shaped heat exchanger pipe assembly of the coil heat exchanger tends to expand in axial direction under the influence of the internal fluid pressure prevailing therein. The solution chosen in the publication in question is to provide a special construction which is not part of the housing, and which comprises ties, an annular bottom plate and a cover plate. The ties, the bottom plate and the cover plate are connected with one another, thereby axially clamping the coil shaped heat exchanger pipe assembly. This assembly is shown in FIG. 3 of the publication in question. The plastic housing consists of a first and a second housing part. The first housing part comprises two connection stubs for passing the two ends of the heat exchanger pipe assembly. The heat exchanger pipe assembly is placed in the first housing part. Thereupon the second housing part is connected with the first housing part and finally, with the help of a crimp connection, the cover plate is connected with an opening edge which is bounded by the plastic housing formed by the first and the second housing part. Further, to protect the plastic housing from the hot gases, in the housing a heat shield is arranged which is situated between the cylindrical surface of the plastic housing parts and the heat exchanger pipe assembly. In the cover plate there is a central opening which is closed off with a burner cover assembly. With the aid of bolt/nut connections the burner cover assembly is bolted to the cover plate, whereby the bolts are under strain of tension in axial direction. Between the burner cover assembly and the cover plate there is a seal which is compressed in axial direction by the tension prevailing in the bolts. Such a heat exchanger is marketed by Sermeta in various sizes, with the designations FHC for central heating and ISO Tankless and ISO Storage for sanitary water heating.
A similar heat exchanger is described in WO 2011/092332 A1 (Rocco Giannoni), an English-language equivalent of which is US2012/0312513 A1. The drawings are of a schematic nature, the description, however, is clear. According to paragraph [0078] the housing may be made from metal, in particular stainless steel. According to paragraph [0079], however, the housing is preferably made from plastic, in particular two plastic halves. The plane in which the two halves are joined together is not clearly and unambiguously disclosed in the description but FIG. 3 seems to suggest that the separating plane coincides with the plane defined by the line I-I in that figure. Of relevance is that paragraph [0088] discloses that when the housing is made from plastic, mechanical means are present for keeping the spiral bundles axially together and that these mechanical means are configured as described in FR 2 846 075 and FR 2 850 451. FR 2 846 075 belongs to the same family as US2006/0196450 A1 already mentioned and described above. The teaching of WO 2011/092332 A1 and US2012/0312513 A1 is therefore the same as that of US2006/0196450, namely, when the housing is made from plastic, use is to be made of a special construction which is not part of the housing, and which comprises ties, an annular bottom plate and a cover plate. The ties, the bottom plate and the cover plate are connected with one another, thereby axially clamping the coil shaped heat exchanger pipe assembly.